In a normal spine, a healthy intervertebral disc can compress about 1 mm when subjected to an axial load of 750 pounds force (3.4 kN). This compression provides the spine with an effective shock absorber against high loads.
The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of articulation weight through a damaged, unstable vertebral joint. One proposed method of managing these problems is to remove the problematic disc and replace it with a prosthetic disc that allows for the natural motion between the adjacent vertebrae (xe2x80x9ca motion discxe2x80x9d).
However, it appears that conventional articulating motion discs do not provide the shock absorbing function of a natural intervertebral disc.
U.S. Pat. No. 6,368,350 (xe2x80x9cEricksonxe2x80x9d) discloses a three-piece articulating motion disc providing two articulation surfaces. The disc comprises a first piece having a curved surface, a second piece having a flat surface, and an intermediate piece (or xe2x80x9ccorexe2x80x9d) having a corresponding curved articulation surface and a corresponding flat articulation surface. Erickson discloses essentially unitary core components that must be made from xe2x80x9cnon-compressiblexe2x80x9d materials. See Erickson at col. 5, lines 49-50. Accordingly, Erickson does not teach a device having the shock absorbing function of a natural intervertebral disc.
U.S. Pat. No. 5,676,701 (xe2x80x9cYuanxe2x80x9d) discloses, in one embodiment, a motion disc having a single articulation surface. This device includes a first component whose inner surface comprises a concave inner portion having a 360xc2x0 circumference and a convex peripheral portion, and an opposing second component whose inner surface comprises a conforming convex inner portion and a convex peripheral portion. The convex/concave contours of the opposing inner portions form a ball-and-socket design that allows unrestricted pivotal motion of the device, while the opposing convex peripheral contours allow flexion/extension bending motion in the range of about 20-30xc2x0. In another embodiment, Yuan discloses a device having two articulation interfaces, wherein one of the above-mentioned components is made in two pieces having opposing flat surfaces that form a translation interface to further provide the prosthetic with a certain amount of translation. See FIG. 9 of Yuan. Yuan discloses essentially unitary components made from essentially incompressible materials, such as ceramics, metals and high density polyethylene. See Yuan at col.6, lines 8-14. Accordingly, Yuan does not teach a device having the shock absorbing function of a natural intervertebral disc.
U.S. Pat. No. 5,507,816 (xe2x80x9cBullivantxe2x80x9d) discloses a three-piece articulating motion disc providing two articulation interfaces and comprises an upper piece having a flat lower surface, a middle spacer having a flat upper surface and a convex lower surface, and a lower piece having a concave upper surface. The articulating convex and concave surfaces form an articulating interface that allows pivotal motion, while the flat surfaces form a translation interface that allows translational motion. Bullivant further teaches that the natural tension of the vertebrae ensures that the vertebrae are biased together to trap the spacer in place, and that the 90xc2x0 extension of the convex and concave surfaces virtually eliminates any chance of the spacer escaping from between the plates under normal pivotal movement of the vertebrae. Bullivant discloses an essentially unitary core component made from polyethylene. See Bullivant at col. 4, line 28. Accordingly, Bullivant does not teach a device having the shock absorbing function of a natural intervertebral disc.
U.S. Pat. No. 5,401,269 (xe2x80x9cButtner-Janzxe2x80x9d) discloses an articulating motion disc having two articulation interfaces and comprises three components: an inferior endplate, a superior endplate, and a core. Both the inferior and superior endplates are metal and have raised bosses with concave spherical surfaces in the center. The core is plastic and has convex surfaces on both the top and bottom which are surrounded by raised rims. Buttner-Janz discloses an essentially unitary core component made from high density polyethylene. See Buttner-Janz at col. 3, line 4. Accordingly, Buttner-Janz does not teach a device having the shock absorbing function of a natural intervertebral disc.
U.S. Pat. No. 5,314,477 (xe2x80x9cMarnayxe2x80x9d) discloses an articulating motion disc having a single articulation interface and comprises three components: an inferior endplate, a superior endplate, and an insert. The inferior endplate functions as a baseplate and has a sidewall forming an open ended channel for reception of the insert. The inner surface of the inferior endplate provides only stationary support for the insert and does not have a motion surface. Since the insert is designed to be locked securely into place within the inferior endplate, the inferior surface of the insert is not a motion surface. The superior surface of the insert includes articulation surface for articulation with the superior endplate. The superior endplate has an inferior articulation surface that articulates with the superior motion surface of the insert, and a superior surface designed for attachment to a vertebral endplate.
French Published Patent Application No. 2,730,159 (xe2x80x9cGermainxe2x80x9d) discloses an articulating motion disc in which the core member has one convex and concave surface. Germain further teaches that the radius of the upper curved surface (3a) of the core member is less than the radius of the lower curved surface (3b) of the core member.
In summary, in all of the conventional motion discs having a core articulation component, the core is unitary and made of a material having a significantly high stiffness. Accordingly, these conventional devices may not fully mimic the shock absorbing function of the natural intervertebral disc.
Therefore, there is a need for a motion device having a core component having two articulation interfaces, wherein the core component has a shock absorbing function mimicking the shock absorbing function of the natural intervertebral disc.
The present inventor has developed a motion disc having a core component that expands laterally under physiologic axial loads, thereby allowing the endplates of the disc to approach each other in order to mimic the shock-absorbing function of the natural disc.
Therefore, in accordance with the present invention, there is provided an intervertebral motion disc comprising:
a) a first prosthetic vertebral endplate comprising:
i) an outer surface adapted to mate with a first vertebral body,
ii) an inner surface having a first articulation surface,
iii) a body portion connecting the inner and outer surfaces,
b) a second prosthetic vertebral endplate comprising:
i) an outer surface adapted to mate with a second vertebral body,
ii) an inner surface comprising a first articulation surface,
iii) a body portion connecting the inner and outer surfaces, and
c) a laterally expandable core member comprising:
a first articulation surface adapted for articulation with the first articulation surface of the first endplate,
a second articulation surface adapted for articulation with the first articulation surface of the second endplate,
wherein the core member is oriented therein to produce a first articulation interface between the first articulation surface of the first endplate and the first articulation surface of the core member, and a second articulation interface between the first articulation surface of the second endplate and the second articulation surface of the core member, and a height H between the outer surfaces of the prosthetic endplates, and
wherein the laterally expandable core member expands laterally when the disc is subjected to axial load of about 750 pounds force (3.4 kN) to reduce the height of the device between 0.5 and 2.0 mm.
Also in accordance with the present invention, there is provided a core member for articulation between first and second prosthetic vertebral endplates, comprising:
i) a first portion having a first articulation surface adapted for articulation with a first articulation surface of the first prosthetic vertebral endplate,
ii) a second portion having a second articulation surface adapted for articulation with a first articulation surface of the second prosthetic vertebral endplate, and
iii) means for transferring an axial load between the prosthetic endplates,
wherein the core member is oriented therein to produce a first articulation interface between the first articulation surface of the first endplate and the first articulation surface of the core member, and a second articulation interface between the first articulation surface of the second endplate and the second articulation surface of the core member.
The disc of the present invention is superior to that of the above-mentioned conventional devices in that at least a portion of the core member expands laterally under physiologic axial loads, thereby allowing the endplates of the disc to approach each other in order to mimic the shock-absorbing function of the natural disc.
In preferred embodiments, the laterally expandable core member expands laterally when the disc is subjected to axial load of about 750 pounds force (3.4 kN) to reduce the height of the device between 0.5 and 2.0 mm.